U.S. patent application number 14/861429 was filed with the patent office on 2017-03-23 for optical interface device having a curved waveguide using laser writing and methods of forming.
The applicant listed for this patent is CORNING OPTICAL COMMUNICATIONS LLC. Invention is credited to James Scott Sutherland.
Application Number | 20170082809 14/861429 |
Document ID | / |
Family ID | 56926356 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170082809 |
Kind Code |
A1 |
Sutherland; James Scott |
March 23, 2017 |
OPTICAL INTERFACE DEVICE HAVING A CURVED WAVEGUIDE USING LASER
WRITING AND METHODS OF FORMING
Abstract
An optical interface device for optically connecting photonic
devices to optical device along with methods of making. The method
includes providing a glass support member that is either monolithic
or laminated. A laser beam is used to write cores in the body of
the support member. The support member includes a bend section and
the cores generally follow the bend section and serve to define
curved optical waveguides. The cores provide strong out-of-plane
optical confinement, thereby allowing for strong bends and
therefore a compact design for the optical interface device.
Inventors: |
Sutherland; James Scott;
(Corning, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CORNING OPTICAL COMMUNICATIONS LLC |
Hickory |
NC |
US |
|
|
Family ID: |
56926356 |
Appl. No.: |
14/861429 |
Filed: |
September 22, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 6/4204 20130101;
C03B 23/0006 20130101; C03B 23/023 20130101; G02B 6/4249 20130101;
G02B 6/4214 20130101; C03C 23/0025 20130101; G02B 6/125 20130101;
G02B 6/4206 20130101; G02B 6/30 20130101 |
International
Class: |
G02B 6/42 20060101
G02B006/42; C03B 23/00 20060101 C03B023/00; C03B 23/023 20060101
C03B023/023; G02B 6/30 20060101 G02B006/30; C03C 23/00 20060101
C03C023/00 |
Claims
1. A method of forming an optical interface device for optically
connecting at least one photonic device to at least one optical
device via at least one optical fiber, comprising: providing a
support member having glass body with a refractive index n.sub.b, a
proximal end, a distal end and an outer surface, wherein the glass
body includes a bend section that includes a curved portion of the
outer surface, with the curved portion of the outer surface being
interfaced with either air or a dielectric coating having a
refractive index n.sub.1<n.sub.b; laser writing at least one
core in the glass body, the at least one core having a refractive
index n.sub.c>n.sub.b and an outer edge closest to the outer
surface and that resides within 10 microns of the curved portion of
the outer surface; and wherein the at least one core defines at
least one curved optical waveguide.
2. The method according to claim 1, wherein the at least one curved
optical waveguide is further defined by a portion of the glass body
adjacent the core and either the air or the dielectric coating.
3. The method according to claim 1, wherein the support member
includes a front-end section, and wherein the front-end section
includes an optical fiber support feature.
4. The method according to claim 1, wherein the core is closest to
the outer surface at the curved portion of the outer surface.
5. The method according to claim 1, wherein the support member has
a vertical dimension LY in the range 2
millimeters.ltoreq.LY.ltoreq.4 millimeters.
6. The method according to claim 1, wherein the at least one core
has a central axis, and wherein the at least one core has a bend
radius RB measured relative to the central axis in the range 1
millimeters.ltoreq.RB.ltoreq.4 millimeters.
7. The method according to claim 1, wherein the at least one core
constitutes a plurality of cores and wherein the proximal ends of
the cores define a first pitch and the distal ends of the cores
define a second pitch.
8. The method according to claim 1, wherein the at least one curved
optical waveguide has a refractive index difference
.delta.=n.sub.c-n.sub.b in the range
0.01.ltoreq..delta..ltoreq.0.02.
9. The method according to claim 1, wherein the at least one core
has a length, and wherein the laser writing is performed such that
a refractive index difference .delta.=n.sub.c-n.sub.b varies over
the length of the core.
10. The method according to claim 3, wherein the support member has
a back-end section further comprising: optically coupling the at
least one optical fiber to the front-end section; and optically
coupling the at least one photonic device to the back-end
section.
11. The method according to claim 13, further comprising optically
coupling the at least one optical fiber to the at least one optical
device.
12. The method according to claim 1, wherein the support member is
laminated structure that includes upper and lower glass sheets, and
wherein the glass body and the outer surface are defined by the
upper glass sheet.
13. The method according to claim 12, wherein the upper and lower
glass sheets are each made of a chemically strengthened glass.
14. The method according to claim 13, wherein: the laser writing of
the at least one core is performed in the upper glass sheet while
the laminated structure is flat, and then bending the laminated
structure to form the at least one curved optical waveguide.
15. The method according to claim 12, wherein the upper and lower
glass sheets sandwich a light-curable adhesive, and wherein the
light-curable adhesive is light cured after the bending of the
laminated structure.
16. The method according to claim 12, wherein the upper glass sheet
includes a front edge, and further comprising optically coupling
the least one optical fiber with the at least one curved optical
waveguide at the front edge of the upper glass sheet.
17. The method according to claim 1, wherein the support member is
monolithic.
18. The method according to claim 17, further comprising forming
the monolithic support member by drawing a glass preform.
19. An optical interface device product formed by the process
comprising: providing a support member having glass body with a
refractive index n.sub.b, a proximal end, a distal end and an outer
surface, wherein the glass body includes a bend section that
includes a curved portion of the outer surface; scanning a laser
beam having a focus spot over the outer surface so that the focus
spot resides within the glass body to form within the glass body a
core having a refractive index n.sub.c>n.sub.b, a curved section
within the bend section, and an outer edge closest to the outer
surface and that resides within 10 microns of the curved portion of
the outer surface, wherein a medium having a refractive index less
than the refractive index n.sub.b resides adjacent the outer
surface; and wherein the core defines at least one curved optical
waveguide.
20. The optical interface device product according to claim 19,
wherein the at least one curved optical waveguide is further
defined by a portion of the glass body adjacent the core and the
medium that resides adjacent the outer surface.
21. The optical interface device product according to claim 19,
wherein the medium is either air or a solid dielectric
material.
22. The optical interface device product according to claim 19,
wherein the support member has either a monolithic structure or a
laminated structure.
23. An optical interface device for optically connecting at least
one photonic device to at least one optical device via at least one
optical fiber, comprising: a support member having a body made of a
single glass material and having a refractive index n.sub.b, a
proximal end, a distal end and an outer surface, wherein the glass
body includes a bend section that includes a curved portion of the
outer surface, with the curved portion of the outer surface being
interfaced with a medium having a refractive index
n.sub.1<n.sub.b; at least one core within the body and defined
by at least one locally densified portion of the single glass
material of body, the at least one core having a refractive index
n.sub.c>n.sub.b and an outer edge closest to the outer surface
and that resides within 10 microns of the curved portion of the
outer surface; and wherein the at least one core defines at least
one curved optical waveguide.
24. The optical interface device according to claim 23, wherein the
at least one curved optical waveguide is further defined by the
medium and a portion of the glass body adjacent the core.
25. The optical interface device according to claim 23, wherein the
medium comprises either air or a dielectric coating
26. The optical interface device according to claim 23, wherein the
support member has a vertical dimension LY in the range 2
millimeters.ltoreq.LY.ltoreq.4 millimeters.
27. The optical interface device according to claim 23, wherein the
at least one core has a central axis, and wherein the at least one
core has a bend radius RB measured relative to the central axis in
the range 1 millimeters.ltoreq.RB.ltoreq.4 millimeters.
28. The optical interface device according to claim 23, wherein the
at least one core constitutes a plurality of cores and wherein the
proximal ends of the cores define a first pitch and the distal ends
of the cores define a second pitch.
29. The optical interface device according to claim 23, wherein the
at least one curved optical waveguide has a refractive index
difference .delta.=n.sub.c-n.sub.b in the range
0.01.ltoreq..delta..ltoreq.0.02.
30. The optical interface device according to claim 23, wherein the
support member has a back-end section and further comprising: the
at least one photonic device optically coupled to the back-end
section; and the at least one optical fiber optically coupled to
the at least one optical device.
31. The optical interface device according to claim 23, wherein the
support member is monolithic and made of a chemically strengthened
glass.
32. The optical interface device according to claim 23, wherein the
support member is laminated structure that includes upper and lower
glass sheets that sandwich an adhesive, and wherein the glass body
and the outer surface are defined by the upper glass sheet.
33. The optical interface device according to claim 32, wherein the
upper and lower glass sheets are each made of a chemically
strengthened glass.
34. The optical interface device according to claim 32, wherein the
upper glass sheet includes a front edge and a back edge, and
further comprising: the least one optical fiber optically coupled
to the at least one curved optical waveguide at the front edge of
the upper glass sheet; and at least one photonic device optically
coupled to the at least one curved optical waveguide at the back
edge of the upper glass sheet so that the at least one optical
fiber and the at least one photonic device are in optical
communication through the at least one curved optical
waveguide.
35. The optical interface device according to claim 34, wherein the
at least one optical fiber is optically coupled to the at least one
optical device.
Description
FIELD
[0001] The present disclosure relates to optical interface devices,
and in particular relates to a method of forming an optical
interface device having a curved waveguide formed using laser
writing, where the optical interface device is used to form optical
interconnections between a photonic device supported by a photonic
integrated circuit board and another optical device such as an
optical fiber connector or an electrical-optical device.
BACKGROUND
[0002] Certain optical and electrical-optical devices have photonic
integrated circuit boards (PICBs) that need to be optically
interfaced (optically coupled) with another optical device so that
optical signals can be received and processed by the device, and
also transmitted from the device to another device or to a back
plane or to the PICB. This often requires plug-in or receptacle
optical connectors that, in some respects, may be similar to
plug-in and receptacle electrical connectors used for electrical
circuit-board components. Alternatively, the optical interface may
be accomplished with a connector end adapted to be permanently (or
semi-permanently) fixed to the PICB to establish and maintain
optical communication with corresponding devices (e.g.,
photodetectors or phototransmitters) on the PICB.
[0003] Because interior space is typically at a premium for most
optical and electrical-optical devices, the optical interface
devices and the PICBs are preferably compact so that they can fit
into tight spaces. For example, multiple PCBs are often arranged in
a stacked configuration within data communications racks, so that
the height of the optical interface devices is preferably as small
as possible while still being able to carry out their function for
transmitting optical signals with suitable optical performance.
[0004] Consequently, there is a need for continuing improvements to
PICB technology that provide for compact configurations while also
providing for high-efficiency optical coupling between the PICB and
another device to which it is optically coupled.
SUMMARY
[0005] An aspect of the disclosure is a method of forming an
optical interface device for optically connecting at least one
photonic device to at least one optical device via at least one
optical fiber, comprising: providing a support member having glass
body with a refractive index n.sub.b, a proximal end, a distal end
and an outer surface, wherein the glass body includes a bend
section that includes a curved portion of the outer surface, with
the curved portion of the outer surface being interfaced with
either air or a dielectric coating having a refractive index
n.sub.1<n.sub.b; laser writing at least one core in the glass
body, the at least one core having a refractive index
n.sub.c>n.sub.b and an outer edge closest to the outer surface
and that resides within 10 microns of the curved portion of the
outer surface; and wherein the at least one core defines at least
one curved optical waveguide.
[0006] An optical interface device product formed by a process that
includes: providing a support member having glass body with a
refractive index n.sub.b, a proximal end, a distal end and an outer
surface, wherein the glass body includes a bend section that
includes a curved portion of the outer surface; scanning a laser
beam having a focus spot over the outer surface so that the focus
spot resides within the glass body to form within the glass body a
core having a refractive index n.sub.c>n.sub.b, a curved section
within the bend section, and an outer edge closest to the outer
surface and that resides within 10 microns of the curved portion of
the outer surface, wherein a medium having a refractive index less
than the refractive index n.sub.b resides adjacent the outer
surface; and wherein the core defines at least one curved optical
waveguide.
[0007] Another aspect of the disclosure is an optical interface
device for optically connecting at least one photonic device to at
least one optical device via at least one optical fiber. The
optical interface device includes: a support member having a body
made of a single glass material and having a refractive index
n.sub.b, a proximal end, a distal end and an outer surface, wherein
the glass body includes a bend section that includes a curved
portion of the outer surface, with the curved portion of the outer
surface being interfaced with a medium having a refractive index
n.sub.1<n.sub.b; at least one core within the body and defined
by at least one locally densified portion of the single glass
material of body, the at least one core having a refractive index
n.sub.c>n.sub.b and an outer edge closest to the outer surface
and that resides within 10 microns of the curved portion of the
outer surface; and wherein the at least one core defines at least
one curved optical waveguide.
[0008] Additional features and advantages are set forth in the
Detailed Description that follows, and in part will be readily
apparent to those skilled in the art from the description or
recognized by practicing the embodiments as described in the
written description and claims hereof, as well as the appended
drawings. It is to be understood that both the foregoing general
description and the following Detailed Description are merely
exemplary, and are intended to provide an overview or framework to
understand the nature and character of the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The accompanying drawings are included to provide a further
understanding, and are incorporated in and constitute a part of
this specification. The drawings illustrate one or more
embodiment(s), and together with the Detailed Description serve to
explain principles and operation of the various embodiments. As
such, the disclosure will become more fully understood from the
following Detailed Description, taken in conjunction with the
accompanying Figures, in which:
[0010] FIG. 1A is a top elevated view and FIG. 1B is a side view of
an example curved or "bent" waveguide support member used in
forming the optical interface device as disclosed herein;
[0011] FIG. 1C is similar to FIG. 1B and shows a different
configuration for the optical optical fiber support features at the
front-end section of the support member, and also shows an example
laser-written waveguide formed therein;
[0012] FIG. 2A is similar to FIG. 1B and shows an optical fiber
operably attached to the front-end section of the support member
using an adhesive material;
[0013] FIG. 2B is a top-down view of the structure of FIG. 2A and
showing an array of optical fibers operably arranged at the
front-end section of the support member;
[0014] FIG. 2C is a close-up side view of the end portion of the
optical fiber, showing how the optical fiber can be supported by
the support features formed in the front-end section of the support
member;
[0015] FIG. 3A is a side view similar to FIG. 2A and illustrates an
example of forming a core in the body of the support member by
laser writing;
[0016] FIG. 3B is similar to FIG. 3A and shows the support member
having a completely formed laser-written core, wherein the core
defines an optical waveguide within the body of the support member
and resides adjacent the outer surface thereof, and wherein the
resulting structure defines an optical interface device;
[0017] FIG. 4A is similar to FIG. 3B and shows the optical
interface device interfaced with a photonic device operably
supported by a PICB;
[0018] FIG. 4B is similar to FIG. 4A, except that the optical fiber
is attached directly to the planar surface at the front-end section
rather than using the support features of FIG. 4A, and wherein the
core includes a front-end section that curves to accommodate the
optical fiber location;
[0019] FIG. 5 is a side view of an example photonic device assembly
that includes a PICB optically coupled to an optical interface
device via the optical interface device of FIG. 4B;
[0020] FIG. 6A is similar to FIG. 4B and illustrates an example of
an optical interface device that includes a 2D optical fiber array,
with two optical fibers being optically coupled to respective
photonic devices via corresponding cores formed in the support
member;
[0021] FIG. 6B is similar to FIG. 6A and illustrates an example of
an optical interface device wherein pairs of optical fibers in the
2D optical fiber array are optically coupled to respective pairs of
photonic devices on the PICB via a corresponding pair of cores
formed in the support member;
[0022] FIGS. 7A through 7D are side views of an example laminated
structure used to form an optical interface device that includes a
curved waveguide, with FIG. 7C showing a core being laser written
into an upper glass sheet before bending and FIG. 7D showing the
final optical interface device structure after bending;
[0023] FIG. 8A is a side view of an example laminated optical
interface device operably interfaced with an optical fiber and with
a photonic device supported by a PICB;
[0024] FIG. 8B is similar to FIG. 5 and is a side view of an
example photonic device assembly that includes a PICB optically
coupled to an optical device via the laminated optical interface of
FIG. 8A;
[0025] FIG. 9 is a schematic side view in the x-y plane of a
laser-written waveguide formed in a glass support member, which is
shown as extending uncurved in the x-direction for ease of
illustration, and wherein the core of the laser-written waveguide
is shown extending from the proximal-end planar surface to the bend
section and varies in size, shape and location along its
length;
[0026] FIGS. 10A, 10B and 10C are y-z cross-sectional views of the
glass support member taken at various section locations A, B and C
shown in FIG. 9, illustrating how the core may vary in size, shape
and location along its length;
[0027] FIGS. 11A and 11B are cross-sectional views similar to FIG.
10C and show two embodiments wherein the outer edge of the core
resides just below the upper surface of the support member at the
bend section (FIG. 11A), and wherein the core intersects the upper
surface at the bend section (FIG. 11B);
[0028] FIG. 12 is similar to FIG. 9 and shows an example transition
of the core from the bend section to the distal-end planar
surface;
[0029] FIGS. 13A through 13C are similar to FIGS. 10A through 10C,
and are y-z cross-sectional views of the glass support member taken
at various section locations A, B and C shown in FIG. 12,
illustrating how the core varies in size, shape and location along
its length;
[0030] FIG. 14 is similar to FIG. 12, and illustrates an example
that includes an overclad layer on the upper surface;
[0031] FIGS. 15A through 15C are cross-sectional views of the
waveguide similar to FIGS. 13A through 13C taken at various section
locations A, B, and C shown in FIG. 14, and show the added overclad
layer;
[0032] FIG. 16A is a cross-sectional view of an example core in the
bend section of the support member, showing an example embodiment
wherein a ridge waveguide structure is formed by adding parallel
grooves in the upper surface and at the sides of the core;
[0033] FIG. 16B is similar to FIG. 16A, and shows a dielectric
material filling the grooves;
[0034] FIG. 17 is similar to FIG. 9 and illustrates an example
where the core has a refractive index that gradually decreases at
the transition to the bend section so that the waveguide at the
bend section has no core and is defined by the overclad layer and
the body of the support member; and
[0035] FIGS. 18A through 18C are y-z cross-sectional views of the
glass support member taken at various locations A, B and C in FIG.
17, showing how the core changes in size and position (FIGS. 18A,
18B) and ultimately disappears at the bend section (FIG. 18C);
DETAILED DESCRIPTION
[0036] Reference is now made in detail to various embodiments of
the disclosure, examples of which are illustrated in the
accompanying drawings. Whenever possible, the same or like
reference numbers and symbols are used throughout the drawings to
refer to the same or like parts. The drawings are not necessarily
to scale, and one skilled in the art will recognize where the
drawings have been simplified to illustrate the key aspects of the
disclosure.
[0037] The claims as set forth below are incorporated into and
constitute part of this Detailed Description.
[0038] Cartesian coordinates are shown in some of the Figures for
the sake of reference and are not intended to be limiting as to
direction or orientation. Likewise, terms such as "vertical" and
"horizontal" are used for the sake of reference and are not
intended to be limiting as to orientation or direction.
[0039] The vertical dimension LY is used to refer to the height of
the support member and the relatively small values of LY as
discussed below define the low-profile nature of the optical
interface devices.
[0040] Monolithic Embodiment
[0041] FIG. 1A is a top elevated view and FIG. 1B is a side view of
an example curved or "bent" waveguide support member ("support
member") 10. The support member 10 has a body 11 that may be made
of any suitable material(s) or construction. By way of example and
not limitation, body 11 may be monolithic and further in an example
is made of a glass with a CTE (Coefficient of Thermal Expansion)
close to the CTE of photonic chip materials (e.g., silicon).
Example glasses include PYREX.RTM. glass, Gorilla.RTM. glass and
WILLOW.RTM. 7740 glass, which may or may not be chemically
strengthened. The body 11 has a bulk refractive index n.sub.b. The
support member 10 includes a proximal end 12 with a planar surface
13, a distal end 14 with a planar surface 15, an upper (or outer)
surface 22 and a lower (or inner) surface 24.
[0042] The support member 10 also includes a bend section 20 that
defines an outer curved portion 32 of upper surface 22 and an inner
curved portion 34 of lower surface 24. Depending on the
application, the y-axis offset of the lower surface 24 from the
planar surface 15 may be greater than a minimum value to provide
clearance for other components located beneath the lower surface
24. Alternatively, lower surface 24 may be co-planar with the
planar surface 15 if clearance for other components is not
required.
[0043] The support member 10 has a front-end section 42 that
includes proximal end 12 and a back-end section 44 that includes
distal end 14. A curved central axis AX runs between the proximal
end 12 and the distal end 14. In an example, bend section 20 has an
associated bend angle .theta. defined by the intersection of a
first plane P1 defined by proximal-end planar surface 13 and a
second plane P2 defined by distal-end planar surface 15. In one
example, the bend angle .theta. can be in the range
45.degree..ltoreq..theta..ltoreq.90.degree., while in other
examples the bend angle can be in the range from
60.degree..ltoreq..theta..ltoreq.90.degree. or
80.degree..ltoreq..theta..ltoreq.90.degree.. For the bend angle of
90.degree. shown in FIGS. 1A and 1B, the proximal-end planar
surface 13 lies in the y-z plane and the distal-end planar surface
15 lies in the x-z plane.
[0044] In an example, front-end section 42 includes an optional
optical fiber support feature 50, which in an example includes
first and second steps 52 and 54 formed in upper surface 22, with
the first step being adjacent proximal end 22 and deeper than the
second step. The first and second steps 52 and 54 serve as an
optical fiber support and strain-relief feature for supporting one
or more optical fibers 100, which are discussed below. The second
step 54 defines a vertical optical fiber stop surface 56, as
explained below.
[0045] The first and second steps 52 and 54 may include other
features, such as a raised ridge 55 or a trench 57, as shown in
FIG. 1C if desired. The ridge and trench features 55 and 57 can
experience some rounding during drawing while allowing the optical
fiber stop surface 56 to remain flat in drawing. The trench 57 also
serves as a reservoir to capture excess optical fiber attachment
material. If the raised ridge 55 interferes with the laser writing
process by distorting the surface profile, it can be easily removed
via a post-draw polishing step.
[0046] FIG. 2A is similar to FIG. 1B and shows an optical fiber 100
operably arranged at the front-end section 42 of support member 10
at fiber support feature 50. When support member 10 is formed using
a drawing process, the drawing process ensures that the optical
fiber stop surface 56 is flat. Thus, as long as the optical fibers
100 are all cleaved to the same length (via for example, precision
laser cleaving and/or polishing), any suitable gap distance between
the bare optical fiber end faces 122 and the optical fiber stop
surface 56 should be acceptably small for acceptable optical
performance. As discussed below, an index-matching material can be
used to fill the gap between the bare optical fiber end faces 122
and the optical fiber stop surface 56 to make the optical coupling
more efficient.
[0047] If the first and second steps 52 and 54 of optical fiber
support feature 50 cannot be accurately formed during the drawing
process, they can be created via post-draw grinding or sawing
operations. In particular, optical fiber stop surface 56 needs to
be smooth and flat to provide a low-loss interface for optical
fiber coupling. If this vertical surface cannot be formed during
drawing due to excessive rounding of surfaces, it may be necessary
to flatten it after drawing, e.g., via diamond sawing or the
like.
[0048] In one embodiment, the support member 10 has a length LX in
the x-direction, a length LY in the y-direction (i.e., a "vertical"
dimension or "height") and a length LZ (i.e., a "width") in the
z-direction. In an example, LX is in the range 4 millimeters (mm)
to 6 mm, LY is in the range 2 mm to 4 mm and LZ is in the range 2
mm to 4 mm. Of course, the concepts disclosed herein may be used
with device having other sizes as desired.
[0049] In an example, support member 10 is formed using glass
drawing techniques known in the art. Glass drawing techniques allow
for the use of a preform that has the same overall shape as the
final support member but larger dimensions. The glass preform is
then drawn to form the reduced-dimension support member 10. In an
example, a glass preform having dimensions of LX'=5 centimeters
(cm), LY'=3 cm and LZ'=40 cm was drawn to form a support member
having corresponding dimensions LX=5 mm, LY=3 mm and LZ=40 mm. In
an example, support member 10 can be polished after it is drawn to
ensure that the front-end and back-end planar surfaces 13 and 15
are truly flat and that any sharp corners are rounded off.
[0050] FIG. 1C shows support member 10 having a laser-written
waveguide 350 formed therein adjacent outer surface 22. The process
of forming laser-written waveguide 350 and its various
configurations and properties are discussed in greater detail
below. With reference to FIG. 1C, outer curved portion 32 of outer
surface 22 at bend section 20 may induce scattering losses in a
laser-written waveguide 350 (dashed line) in regions where debris
comes in contact with this surface. Thus, in an example, outer
surface 22 (and especially at outer curved portion 32 of bend
section 20) interfaces with a low-index coating 23 of refractive
index n.sub.1<n.sub.b to reduce or prevent such optical loss by
maintaining a debris-free surface. In an example, low-index coating
23 is made of a polymer. In another embodiment, at least the outer
curved portion 32 of outer surface 22 interfaces directly (i.e.,
has a boundary interface) with an outside environment 40, which in
an example is air.
[0051] Once the support member 10 is formed or otherwise obtained
and at least one laser-written waveguide 350 formed therein, it is
used to form an optical interface device 400, which is introduced
and discussed below. FIG. 2B is a top-down view of support member
10 showing a plurality of optical fibers 100 operably disposed on
front-end section 42 of support member 10. FIG. 2C is a close-up
side view of the optical fiber 100 and front-end section 42 of
support member 10. Optical fiber has a first end portion 101a and
an opposite send end portion 101b (see FIG. 5, introduced and
discussed below).
[0052] As best seen in FIG. 2C, each optical fiber 100 includes a
core 102 and a cladding 104 that surrounds the core. The core 102
and cladding 104 may be made of glass and are configured to form an
optical waveguide, but other suitable materials are possible. The
cladding 104 is surrounded by a non-glass coating 110, such as a
polymer (e.g., polyurethane acrylate). When a portion of coating
110 is removed ("stripped") from optical fiber 100 at end portion
101a, the result is an exposed glass section 120, which is referred
to in the art as a "bare fiber section." Each optical fiber 100 to
be interfaced with support member 100 is processed so that it
includes a first end portion 101a having bare fiber section 120,
which has an end face 122. In the discussion below, reference is
sometimes made to a single optical fiber 100 for ease of
discussion. The optical fiber 100 has a central axis AF that runs
down the center of core 102.
[0053] The optical fiber 100 is disposed on front-end section 42 of
support member 10, e.g., at optical fiber support feature 50, with
its bare fiber section 120 residing on the second step 54 and while
the rest of the the end portion resides on the first step 52. In
this configuration, end face 122 of bare fiber section 120 resides
immediately adjacent or in intimate contact with vertical optical
fiber stop surface 56 defined by second step 54.
[0054] The optical fiber 100 can be secured to front-end section 42
of support member 10 using, for example, an adhesive material 140,
such as epoxy. In an example, adhesive material 140 is light
curable, e.g., with ultraviolet (UV) light. Other optical fibers
100 can be added to support member 10 in the same manner. In an
example, adhesive material 140 is index-matched to the optical
fiber 100 and to the bulk refractive index n.sub.b of support
member 10 to reduce optical loss.
[0055] Once the desired number of optical fibers 100 are added to
support member 10 as described above, the next step involves
forming at least one laser-written waveguide 350. This involves
forming at least one high-index core ("core") 300 in body 11 of the
support member. FIG. 3A is similar to FIG. 2A shows an example of
forming core 300 in body 11 of support member 10 by laser writing.
The laser writing is accomplished using a laser beam 220 from a
laser system 222. In an example, laser beam 220 is brought to a
focus 224 (e.g., with a focusing lens 230 such as a microscope
objective) within body 11 of support member 10 at a select focus
depth relative to top surface 22. The focus spot 224 has a select
size FS, e.g., a diameter. The specific wavelength, pulse duration
(i.e., pulse width) and pulse energy of laser beam 220 need to form
core 300 depends on the particular glass used.
[0056] In an example, laser beam 220 is pulsed (e.g., comprises
femtosecond pulses) and has sufficient energy at focus spot 224 to
locally change the bulk refractive index n.sub.b of body 11 to a
core refractive index n.sub.c, wherein n.sub.c>n.sub.b. By
scanning laser beam 220 (as shown by arrow AR1 in FIG. 3A), core
300 is formed within body 11 of support member 10. The dashed-line
section of core 300 represents the portion of the core 300 to be
formed as the laser beam 220 continues scanning. As discussed
below, core 300 can vary in size, shape and position (relative to
outer surface 22) along its length. In an example, a refractive
index difference .delta.=n.sub.c-n.sub.b is between 0.01 and 0.02
(i.e., between 1% and 2%). In another example, .delta.<0.01. The
refractive index difference .delta. can also vary along the length
of core 300, as discussed below. In one example discussed below in
connection with FIG. 17, the refractive index .delta.=0 over a
portion of core 300 at bend section 20.
[0057] While core 300 is shown in FIG. 3A and in other Figures as
having a sharp boundary, this is for ease of illustration. In
practice, refractive index of core 300 can vary laterally, i.e.,
can have a graded index, in which case the "edge" of the core is
not well-defined. In such a case, an edge for core 300 can be
denoted using any one of a number of criteria, such as the 1/e or
1/e.sup.2 point of the change in refractive index n.sub.c relative
to its on-axis value.
[0058] FIG. 3B shows the completed laser-written core 300, which in
the example shown extends from optical fiber stop surface 56 to the
distal-end planar surface 15. The laser writing of optical
waveguides in bulk glass is known in the art and is described in,
for example, U.S. Pat. Nos. 6,573,026 and 6,977,137, which are
incorporated by reference herein. The core 300 includes a front-end
section 312 at front end 42 of support member 10, a curved section
320 at bend section 20 of the support member, and a back-end
section 324 at back-end section 44 of the support member. The core
300 can be generally said to be "curved" because is substantially
follows the outer curved portion 32 of outer surface 22 of support
member 10.
[0059] While not wishing to be bound by theory, it is believed that
the change in the refractive index induced in body 11 by the laser
writing process that defines core 300 may be caused by a localized
densification of the bulk glass or by the the generation of color
centers. It may be that both of these phenomena occur at the same
time to cause the localized refractive index change. In either
case, core 300 is formed without having to add material (e.g., a
dielectric material, index-changing dopants, etc.) to body 11
because the refractive index change is induced directly into the
body by the photons in laser beam 220, wherein the photons are not
considered to be "material." Thus, core 300 is formed by a
localized change in the refractive index of the material that makes
up body 11 so that the body remains monolithic after the formation
of core 300. The same is true in the example of the laminated
embodiment discussed below, wherein the body 611 of the upper glass
sheet 610 remains monolithic after the formation of core 300. In an
example, core 300 is thus said to be formed as a locally densified
portion of the single glass material that constitutes body 11 or
body 611.
[0060] FIGS. 3A and 3B include a photonic device 250 disposed
adjacent distal-end planar surface 15. The photonic device 250 is
operably supported by a PICB 260, introduced and discussed below,
and is shown in FIGS. 3A and 3B to illustrate the position of the
photonic device when support member 10 is used as an optical
interface device 400. The photonic device 250 includes a
photoactive element 252, such as a light receiver (e.g.,
photodetector) or light emitter (e.g., light source). The
photoactive element 252 has a dimension (e.g., a diameter) DP. The
photonic device 250 is supported by a PCIB 260 having an upper
surface 262. Alternatively, photonic device 250 may be a passive
photonic device, such as a waveguide grating vertical coupler that
is fabricated on the surface of PICB 260.
[0061] The configuration of core 300 is defined in part by the
location of the end face 122 of optical fiber 100 on vertical wall
56, the size of core 102 and its (y, z) position at the fiber end
face, and the (x, z) position of the photoactive element 252 of
photonic device 250 at distal-end planar surface 15. Each core 300
has a central axis AC and is formed so that its central axis is
aligned with core 102 (i.e., with optical fiber axis AF) and with
photonic device 250, and in particular with photoactive surface
252. In an example, both the core 102 of optical fiber 100 and the
photonic device 250 reside adjacent the outer surface 22, which
allows core 300 to be formed at or immediately adjacent to the
outer surface.
[0062] The core 300 thus has a bent or curved shape that generally
follows the contour of outer surface 22 at least at outer curved
portion 32. The core 300 may have a diameter DC that can vary along
its length. The core 300 also has an outer edge 302 that resides
closest to outer surface 32. The depth of the core central axis AC
relative to outer surface 22 is given by dC.sub.A, while the depth
of the core to the outer edge 302 is given by dC.sub.E. The core
depths dC.sub.A and dC.sub.E can vary along the length of core 300.
In an example, the core depths dC.sub.A and dC.sub.E can be
substantially constant in bend section 20.
[0063] The core 300 has a cross-sectional area AR.sub.c that can
vary along the length of the core. The cross-sectional shape of
core 300 can be but need not be circular, and can be oval, oblate,
etc. In an example, the core diameter DC substantially matches that
of the diameter of the core 102 of optical fiber 100 at optical
fiber stop surface 56 (e.g., DC is about 8 to 10 microns) and
substantially matches the diameter DP of photoactive element 252 at
distal end 14 (e.g., DP is about 20 microns to 60 microns).
Consequently, the shape and/or size of the core 300 can be matched
to the interfacing component for improved optical coupling.
[0064] In an example, the cross-sectional area AR.sub.c of core 300
is greater at bend section 20 than at the proximal and distal ends
12 and 14 of support member 10, e.g., is between about 20% to 500%
greater than at proximal end 12.
[0065] Thus, in an example, once the (y, z) position of each
optical fiber core 102 of optical fibers 100 is established and the
(x,z) position of each photonic device 250 is established, the
cores 300 are written by scanning laser beam 220, starting for
example, from each of the (y, z) positions of optical fiber cores
102 and continuing to the corresponding (x, z) position of the
corresponding photonic device 250. In one example, this
laser-writing process results in an array of cores 300 that lie
substantially in respective x-y planes that have different z
positions. In an example, there can be some variation of the cores
300 from residing strictly in respective x-y planes, e.g., when
some variation in the pitch of the cores at the proximal and distal
ends of support member 10 requires an excursion of the core in the
z-direction.
[0066] In an example, front-end section 312 of core 300 gradually
transitions from the (y, z) location of the optical fiber core 102
up to near outer curved portion 32 of outer surface 22 at bend
section 20. Once cores 300 are close to outer surface 22 (e.g.,
within 10 microns) at bend section 20, the scanning laser beam 220
continues to write cores 300 around the curved portion 32 of outer
surface 22 while maintaining a constant core depth dC.sub.E from
outer surface 22, thereby forming the curved section 320 of core
200. The laser writing of cores 300 continues from the curved
section 320 and forms back-end section 324 that terminates at
distal-end planar surface 15. As discussed above, cores 300 have a
first pitch at optical fiber stop surface 56 or proximal-end planar
surface 12 that may match the optical fiber pitch, and a second
pitch at distal-end planar surface 15 that may match the pitch of
the photonic devices 250. This configuration aligns cores 300 with
optical fibers 100 at the core front-end section 312 and aligns the
cores with the photonic devices 250 on PICB at the back-end section
324.
[0067] Each core 300 serves to define one of the aforementioned
waveguides 350 in body 11 of support member 10. Each waveguide 350
is curved and includes a core 300, with the waveguide cladding
defined by the portion of lower-index body 11 that resides adjacent
the core, as well as the lower-index air of index n.sub.a.apprxeq.1
or a low-index coating 23 of index n.sub.1 that resides adjacent
upper surface 22. The waveguide 350 in FIG. 3B is shown with a
guided light wave ("guide wave") 352 traveling therein in the
direction from distal end 14 toward proximal end 12 and formed by
light 354 emitted from photoactive element 252 and coupled into the
waveguide.
[0068] As discussed above, the difference .delta. between the core
refractive index n.sub.c of core 300 and the bulk refractive index
n.sub.b of body 11 can be about 1% to about 2%. This index
difference generally makes for a weakly guiding waveguide 350.
However, by locating core 300 at or adjacent outer surface 22
(e.g., with the core outer edge 302 residing within 10 microns
thereof), the low-index coating 23 or the air that resides adjacent
the outer surface 22 also constitutes part of the cladding for the
waveguide 350. In other words, the propagation of guided wave 352
in waveguide 350 is affected by the proximity to the low-index
coating 23 or the air adjacent the outer surface. This creates a
relatively large index difference, e.g., about 3% to 1% with
respect to the low-index coating refractive index n.sub.1 or about
30% to 33% with respect to air. Thus, waveguide 350 is actually a
relatively strong waveguide with respect to bend section 20. This
allows for the central axis AX of support member 10 at bend section
20 to have a relatively small radius of curvature RB (FIG. 3B),
e.g., in the range from 1 mm RB 4 mm.
[0069] As noted above, the waveguide core 300 can be substantially
circular in cross-section and can have a diameter DC that
substantially matches that of the optical fiber core 102 at the
front-end section 42 and that substantially matches a diameter of
photoactive element 252 at back-end section 44. This allows for
waveguide 350 to have substantially matched mode-field diameters at
its two ends.
[0070] The exact location of outer surface 22 may be determined
prior to or during the laser writing of core 300 by using a
suitable instrument, such as a scanning laser confocal
profilometer, which can determine the surface location with high
precision (e.g., 0.1 micron). The core 300 can be written by laser
beam 220 so that it is formed slightly below curved portion 32 of
outer surface 22 in bend section 20. For example, the core outer
edge may be located at a depth dC.sub.E that is 0 to 10 microns
below outer surface 22 at bend section 20, or in another embodiment
4 microns to 8 microns below the outer surface.
[0071] As noted above, the close proximity of the curved section
320 of core 300 to the curved portion 32 of outer surface 22
creates a relatively large index of refraction difference that
provides strong optical confinement, thereby enabling low-loss
guiding of guide wave 352 through waveguide 250 at bend section 20,
where it follows a small bend radius RB (e.g., 2 mm to 3 mm). A
smooth outer surface 22 formed by the high temperature draw process
or by subsequent polishing minimizes scattering losses, as can the
use of the aforementioned low-index outer coating 23. In an
example, the curved section 320 of core 300 is substantially
concentric with the curved portion 32 of outer surface 32, i.e.,
the core depth dC.sub.A is substantially constant over the curved
portion 32 of bend section 20 of support member 10.
[0072] When multiple optical fibers 100 are employed, multiple
cores 300 are formed having a first pitch that matches the pitch of
the optical fibers at front-end section 42 and a second pitch that
matches the pitch of the photonic devices 250 at back-end section
44. If needed, the cores 300 can be displaced away from outer
surface 22 at the back-end section 44 to accommodate in-board
locations of photonic devices 250.
[0073] FIG. 4A is similar to FIG. 3B and shows support member 10
with optical fiber 100 operably disposed thereon at front-end
section 42, and with waveguide 350 formed therein and operably
arranged relative to PICB 250 and photonic device 250 thereon. The
support member 10 with waveguide 300 formed therein constitutes an
optical interface device 400 that allows light 354 to communicate
between optical fiber 100 and photonic device 250. As shown in FIG.
2B, multiple optical fibers 100 can be so arranged at front-end
section 42 of support member 10.
[0074] FIG. 4B is similar to FIG. 4A and illustrates another
embodiment of support member 10 wherein the support member does not
include optical fiber support feature 50. Instead, the optical
fiber(s) 100 is/are directly connected to planar surface 13 at
proximal end 12 using, for example, adhesive material 140, which
also provides for strain relief. Note that front section 312 of
core 300 includes an offset 313, which in an example is a gradual
(i.e., adiabatic) bend that brings the core from near the outer
surface 22 deeper into the body 11 in order to align with core 102
of optical fiber 100. A sufficiently gradual (adiabatic) bend in
offset 313 allows for the guide wave 352 to continue to propagate
through waveguide 350 without substantial loss.
[0075] With continuing reference to FIG. 4B, optical interface
device 400 is aligned with photonic integrated devices 250
supported by PICB 260 and then attached to the PICB using, for
example, adhesive material 140.
[0076] FIG. 5 is a side view of an example photonic device assembly
500 that includes PICB 260 optically coupled to an optical device
510 via the optical interface device of FIG. 4B. The optical device
510 includes an optical fiber connector 520 configured to receive
respective second ends 101b of optical fibers 100. The photonic
device assembly 500 includes a package 530, such as a quad
small-form-factor pluggable (QSFP) package.
[0077] FIG. 6A is similar to FIG. 4B and illustrates an example
optical interface device 400, wherein the optical fibers 100 are
arranged in a 2D array at proximal-end planar surface 13 of
front-end section 42, with the 2D array having a top row 100T and a
bottom row 100B. In FIG. 6A, cores 300 are formed to connect to
each of the optical fibers 100 by including core paths within
front-end section 312 of core 300 that extend deep into body 11 of
support member 10 to reach the bottom row 100B of optical fibers.
The cores 300 for a given pair of optical fibers 100T and 100B
(i.e., with the same z-coordinate but different y-coordinate)
diverge in the z-coordinate direction to be spaced by a fixed pitch
(e.g., 125 microns) as they ultimate converge to share the same
offset distance from the smooth glass curved surface, with one core
residing "behind" to other in FIG. 6A. The pair of cores 300 travel
in parallel adjacent outer surface 22 around the bend section and
down to distal-end planar surface 15 and to respective photonic
devices 250 with different z-locations, or to a single photonic
device.
[0078] FIG. 6B is similar to FIG. 6A and shows a 2D array of
photonic devices 250 on PICB 260 along with the 2D array of optical
fibers 100. In FIG. 6B, the cores 300 associated with a given pair
of optical fibers with the same z-coordinate but different
y-coordinate respectively optically couple to photonic devices 250
having the same z-coordinate but different x-coordinate. Here, the
paths of the cores 300 simultaneously experience convergence of
their y-coordinates while their z-coordinates diverge, so that they
reside close together at constant pitch adjacent to outer surface
22 as they travel around bend section 20 in parallel and then
diverge again in back-end section 324 to align with respective
photoactive elements 252 on one or more photonic devices 250 on
PICB 260.
[0079] Laminated Embodiment
[0080] FIGS. 7A through 7D are side views of an example support
member 10 defined by a laminated structure 600. With reference
first to FIG. 7A, the laminated structure 600 includes a front or
proximal end 602, a back or distal end 604 and upper and lower thin
glass sheets 610 and 620 that sandwich an uncured adhesive material
640 that is, for example curable using ultraviolet light. The upper
glass sheet 610 includes a body 611 with top and bottom surfaces
612 and 614, a front edge 616 and a back edge 618. The lower glass
sheet 620 has top and bottom surfaces 622 and 624. The top surface
612 of upper glass sheet 610 is also referred to as an outer
surface. In an example, the upper glass sheet 610 is shorter in the
x-direction than the lower glass sheet 620. The laminated structure
600 includes a support block 650 attached (e.g., via an adhesive
640) to the bottom surface 624 of lower glass sheet 620 adjacent
back end 604.
[0081] The difference in size of the upper and lower glass sheets
610 and 620 defines a ledge 630 at the front end 602 of laminated
structure 600. The edge 616 of upper glass sheet 610 defines a
fiber stop surface. With reference now to FIG. 7B, a first end 101a
of optical fiber 100, which includes a bare fiber section 120, is
disposed on ledge 630 and fixed thereto, e.g., via adhesive
material 140. The end face 122 of bare fiber section 120 resides
immediately adjacent edge or fiber stop surface 616. Here, adhesive
material 140 can be UV-curable or thermally curable. If UV-curable,
shielding can be used to avoid curing the sandwiched adhesive
material 640, which in an example can be the same as adhesive
material 140.
[0082] FIG. 7C is similar to FIG. 7B, except that it shows laser
beam 220 being scanned over laminated structure 600 so that focus
spot 224 forms core 300 in body 611 of upper glass sheet 610. The
core 300 is formed close to or at outer surface 612 of upper glass
sheet 610 and can include front section 312 having an offset 313
wherein the core gently transitions deeper into body 611 of the
upper glass sheet so that it aligns with optical fiber core 102 at
edge 616. Multiple cores 300 can be formed in the same manner and
with a first pitch at front edge 616 to match that of optical
fibers 100 and a second pitch at back edge 618 to match the
anticipated locations of multiple photonic devices on PICB 260. As
with the above embodiment that employed the monolithic support
member 10, the (y, z) positions of optical fiber cores 102 at edge
616 can be established using known techniques and devices, such as
a vision system.
[0083] In an example, the upper and lower glass sheets 610 and 620
are made of a flexible glass, such as Corning.RTM. Willow.RTM.
glass. Other chemically strengthened, flexible glasses can also be
used. In an example, the upper and lower glass sheets 610 and 620
have a coefficient of thermal expansion (CTE) that is closely
matched to silicon. In an example, the upper and lower glass sheets
610 and 620 have a thickness in the range of 40 microns to 60
microns. Such thin glass sheets can be cut into rectangular shapes
via mechanical or laser cleaving operations. Alternatively, such
thin glass sheets can be formed by a glass drawing process. The
uncured adhesive 640 helps hold the upper and lower glass sheets
610 and 620 together during assembly.
[0084] The cores 300 define waveguides 350, wherein a portion of
body 611 of top glass sheet 610 defines a portion of the cladding
and wherein the air or a low-index coating 613 that resides
adjacent top or outer surface 612 defines a portion of the cladding
(see FIG. 8A).
[0085] In another embodiment, at least a portion of cores 300 can
be formed prior to attaching optical fibers 100. In this case, the
laser writing process can be used to form front-end section 312 so
that an optical interconnection is made between the optical fiber
cores and the previously fabricated waveguides.
[0086] If the laser writing process for forming cores 300 would
inadvertently cure the uncured adhesive material 640 sandwiched
between upper and lower glass sheets 610 and 620, it may be
necessary to alter the assembly order so that the uncured adhesive
is applied after the formation of the cores. In this case, optical
fibers 100 can be directly attached to the upper glass sheet 610
prior forming the cores 300 by laser scanning and optionally
mechanically stabilizing the structure, e.g., by an additional
glass cover sheet (not shown). Then after cores 300 are formed by
scanning laser beam 220, the lower glass sheet 620 is attached to
upper glass sheet 610 using uncured curable adhesive 640.
[0087] Once optical fibers 100 are operably disposed on laminated
structure 600 and cores 300 are formed in body 611 of upper glass
sheet 610, the upper and lower glass sheets are bent. The bending
can be performed in any one of a number of different ways,
including by using a jig that supports the laminated structure and
that allows the structure to be placed in and then held in a bent
position.
[0088] After bending laminated structure 600, the curable adhesive
640 is cured, e.g., by illuminating with light of the curing
(actinic) wavelength, such as a UV wavelength. This curing locks
the upper and lower glass sheets 610 and 620 into a desired curved
shape that includes bend section 20. At this point, bottom edge 618
can be polished to provide a flat optical interface for coupling to
photonic devices 250 on PICB 260. In addition, low-index coating
613 can be added to top surface 612 of upper glass sheet 610.
[0089] FIG. 8A is a side view of an example laminated optical
interface device 400 formed by laminated structure 600 described
above, wherein the optical interface device is operably coupled to
optical fiber 100 at front end 602 and to photonic device 250 on
PICB 260 at back end 604. The embodiment of laminated structure 600
includes low-index coating 613 over a portion of top surface 622 at
bend section 320.
[0090] FIG. 8B is similar to FIG. 5 and is a side view of an
example photonic device assembly 500 that includes PICB 260
optically coupled to an optical device 510 via the example
laminated optical interface device 400 of FIG. 8A. The optical
device 510 includes an optical fiber connector 520 configured to
receive respective second ends 101b of optical fibers 100. The
photonic device assembly 500 includes a package 530, such as a quad
small-form-factor pluggable (QSFP) package.
[0091] Laser Writing Process Considerations
[0092] The embodiments described herein utilize laser writing for
creating localized refractive index changes in glass to create a
curved waveguide in the support member. This process can also
induce stresses that lead to surface damage when the cores are
located at or near the glass surface, especially at high laser
powers.
[0093] In an example, the laser writing of waveguides very close
(e.g., within 10 microns) of the glass surface is done with
relatively low power to avoid inducing surface damage. However,
this results in a low waveguide index contrast. This is normally
undesirable in optical interconnections because it makes the
waveguides highly sensitive to bend losses. To avoid bend losses,
bend radii must be increased, resulting in large and often
impractically sized devices.
[0094] For low-profile optical interconnection applications as
discussed herein, the optical waveguide only requires strong
confinement in the out-of-plane direction (i.e., moving towards the
outside of the bend). As the optical waveguide traverses the glass
bend, it experiences little or no bending in the lateral direction
parallel to the axis of the curve. Consequently, extremely weakly
guiding and expanded beam solutions in the lateral direction are
acceptable for low-loss light propagation around the bend
section.
[0095] Standard single-mode SMF-28 waveguides employ a step-index
core with a core relative refractive index .DELTA.=0.36%. The
resulting guiding condition is relatively weak, since a significant
fraction of the optical field exists as an evanescent wave outside
the core region. By reducing the relative refractive index further,
the waveguide can be made extremely weakly guiding. In this case,
the width of the guided wave increases as the optical field extends
even farther beyond the core as an evanescent wave. The width of
the optical fiber core can also be increased if necessary without
introducing higher order guided modes. As long as the core diameter
increases and decreases are performed gradually, light will remain
only in the fundamental mode of the guided even if higher order
modes are supported by the waveguide.
[0096] A modest core diameter increase may be desirable to reduce
core/cladding index perturbation losses at the core/cladding
interface. In this case, increasing the core diameter reduces the
field intensity at the core/cladding interface, which in turn
reduces the field overlap with index profile perturbations that
would otherwise increase optical loss via scattering.
[0097] A small bend in the optical waveguide is equivalent to
application of a minor tilt to the index profile. If the core
region relative refractive index .DELTA. is small enough, the large
portion of light guided in the core can "spill over" into the
cladding, exciting a large number of cladding modes. These guided
cladding modes can have very low overlap with the original unbent
core mode, constituting an undesirable optical loss out of the
guided core region. To avoid loss and the related effective tilting
of the refractive index profile, optical waveguides that are
extremely weakly guided (e.g., with an index difference<0.1%)
need to remain straight in the plane of the weakly guiding index
contrast.
[0098] FIG. 9 is a schematic side view in the x-y plane of a
laser-written core 300 that defines waveguide 350 formed in glass
support member 10, which is shown as extending uncurved in the
x-direction only (i.e., without showing the bend) for ease of
illustration. FIGS. 10A, 10B and 10C are y-z cross-sectional views
of the glass support member taken at various locations A, B and C
shown in FIG. 9. Though the discussion below is directed to the
monolithic example of support member 10, the discussion applies
equally to the laminated example of support member 10.
[0099] The cross-sectional view of FIG. 10A taken at location A is
for the straight front-end section 312 of core 300 of waveguide
350, which is weakly guiding. In front-end section 312, core 300
has a relatively large core depth dC.sub.A. The core depth dC.sub.A
of core 300 may be selected to ensure proper alignment to core 102
of optical fiber 100, as discussed above. For example, if bare
fiber section 120 of an SMF-28 optical fiber 100 is attached
directly to the proximal-end planar surface 13 of support member
10, the central axis AX of core 300 can be located relative far
below upper surface 22 in front-end section 312, e.g., with a core
depth dC.sub.A between 50 microns and 100 microns. If a glass
alignment block or other strain relief fixture is attached to the
array of optical fibers 100 where it interfaces with support member
10, core 300 can be located even deeper within the body 11 of the
support member at front-end section 312, e.g., with a core depth
dC.sub.A between 300 to 500 microns from upper surface 22.
[0100] With reference again to FIG. 9 and to FIGS. 10A through 10C,
core 300 is formed so that is gradually widens and moves towards
upper surface 22 as it approaches bend section 20, so that the core
depth dC.sub.A decreases. Further, the intensity of laser beam 220
is reduced during the writing of core 300 so that the core
refractive index n.sub.c decreases towards bend section 20. Core
300 may also be made wider in the direction parallel to the outside
surface of the bend, i.e., in the z-direction.
[0101] As core 300 continues into bend section 20, the core depth
dC.sub.A decreases even further so that waveguide 350 resides
immediately adjacent upper surface 22 and becomes extremely weakly
guiding (FIG. 10C). The transition of the size, shape and position
of core 300 shown in FIGS. 8A through 8C can take place over a
relatively short distance, such as 300 to 400 microns. The guided
mode 352 traveling in bend section 20 will be displaced upward
(i.e., in the y-direction) toward upper surface 22, with strong
confinement provided in this direction by the relatively large
difference between the refractive index n.sub.c of core 300 and
that of the air (or low-index coating 23) above and which defines a
substantial portion of the cladding of waveguide 350 in the
y-direction on the upper side of the waveguide.
[0102] The strength of the optical confinement of waveguide 350 at
bend section 20 may be moderated by using the aforementioned
low-index coating 23, such as a polymer optical coating. In an
example, the index of refraction n.sub.1 of low-index coating 23 is
1<n.sub.1<n.sub.b, where n.sub.b is the bulk refractive index
of body 11 of support member 10, while the refractive index of air
is taken to be 1, as noted above. The low-index coating 23 can be
used to protect the upper surface 22 of support member 10 from
damage and/or contamination that might lead to scattering losses.
The low-index coating layer 23 can also reduce the optical
scattering loss in bend section 20.
[0103] FIGS. 11A and 11B are cross-sectional views similar to FIG.
10C and show two embodiments wherein the outer edge 302 of core 300
resides just below upper surface 22 of support member 10 at bend
section 20 (FIG. 11A), and wherein the core intersects the upper
surface (FIG. 11B) at the bend section. It may be desirable to form
core 300 as shown in FIG. 11B to minimize tensile stresses and
associated damage at the upper surface 22.
[0104] FIG. 12 is similar to FIG. 9, and FIGS. 13A through 12C are
similar to FIGS. 10A through 10C, and show an example of the
transition of core 300 from bend section 20 to distal-end planar
surface 15. The transition shown is substantially the reverse of
that from proximal-end planar surface 14 to bend section 20.
[0105] FIG. 14 is similar to FIG. 12, and FIGS. 15A through 15C are
similar to FIGS. 13A through 12C, and show the addition of an
overclad layer 623 on upper surface 22 between distal end 14 and
bend section 20 where core 300 is generally straight. The overclad
layer 623 is designed to aid the transition from surface to deeper
optical propagation of guided mode 352. The overclad layer 623 has
an index of refraction n.sub.2 that is more closely matched to the
index of refraction n.sub.b of support member 10 than the low-index
coating 613. The overclad layer 623 reduces the depth dC.sub.A that
core 300 must descend into body 11 to avoid the influence of the
surface confinement. By reducing the required core depth dC.sub.A,
the length of the transition region of back-end section 324 after
bend section 20 can be reduced, thereby reducing the overall height
of the optical interconnection. The overclad layer 623 can be used
in conjunction with low-index coatings 23 or 613, with the
low-index coating disposed atop the overclad layer.
[0106] If the laser writing process can create optical waveguides
with a higher index of refraction near the surface of the glass, it
may not be necessary to transition to a deeper optical waveguide
with higher relative refractive index .DELTA. for efficient
coupling to small diameter active optical devices. Alternatively,
if the active optical device requires a larger diameter interface
(e.g., for an optical grating coupler interface that provides
lateral misalignment tolerance), it may be desirable to utilize a
larger diameter and extremely weakly guiding optical waveguide for
the interface. In this case, the need for an optical waveguide
taper structure may not be required, or it may be foreshortened to
reduce the length of the straight section, and therefore the
overall height of the optical interconnection.
[0107] FIG. 16A is a cross-sectional view of an example core 300 in
bend section 20 of support member 10 showing an example embodiment
wherein a ridge waveguide structure is formed by adding parallel
grooves 700 in upper surface 22 and at the sides of the core. The
grooves 700 can be configured to provide strong lateral confinement
for the optical waveguide 350 the guided wave 352 propagates in
bend section 20. The parallel grooves 700 can be laser written,
e.g., using laser beam 220 with relatively high optical power. In
bend section 20, the guided wave 352 is biased upwards towards the
air-glass interface, so that additional out-of-plane refractive
index confinement is necessary moving into the substrate. The
grooves 700 prevent guided mode 352 from diffracting laterally,
i.e., parallel to the plane of the glass surface, by providing a
large refractive index contrast at the groove surface.
[0108] FIG. 16B is similar to FIG. 16A and illustrates an example
wherein grooves 700 can be filled with a dielectric material 702
that is closely matched but has a refractive index n.sub.3 that is
lower than the bulk refractive index n.sub.b of support member 10
for weak guiding of light in and potentially single-mode operation.
Alternatively, the grooves 700 can be left unfilled as shown in
FIG. 16A, especially if their walls are sufficiently smooth. If the
walls of grooves 700 are not sufficiently smooth after laser
fabrication, they can be smoothed thereafter, e.g., by exposure to
an acid etch. Smooth walls for grooves 700 may also be obtained by
locally damaging the glass, followed by exposure to an acid that
preferentially etches regions that have experienced laser
damage.
[0109] Lateral confinement in the bent substrate region may also be
achieved by selectively applying the aforementioned overclad layer
623 on upper surface 22 at bend 20. For example, a narrow overclad
layer may 623 be applied and lithographically patterned after laser
writing and prior to glass substrate bending, if such is
required.
[0110] FIG. 17 is similar to FIG. 9 and FIGS. 18A through 18C are
similar to FIGS. 10A through 10C and show respective
cross-sectional views of an example support substrate 10 for the
portion from proximal end 12 to bend section 20. The core 300 is
initially deep in body 11 of support member 10 at and near proximal
end 12 in front-end section 312, and and gradually moves toward the
upper surface 22 at bend section 20. Since the patterned overclad
layer 623 provides in-plane lateral alignment while the bend
section 20 provides out-of-plane lateral alignment, core 300 is no
longer needed to form waveguide 350. The waveguide index of
refraction can be gradually reduced from a weakly guiding structure
(FIG. 18A) to no index of refraction delta (FIG. 18C) once the
patterned overclad layer 623 is reached. This configuration for
core 300 can be achieved by gradually decreasing the amount of
power in laser beam 220 during the laser writing of the core.
[0111] The transition of core 300 from distal end 14 to bend
section 20 can be identical or substantially the same, i.e., core
300 is absent at bend section 20 and then its refractive index
gradually increases until the core refractive index is reached for
the back-end section 324 of the core that extends to distal end 14.
The overclad layer 623 extends only as far as necessary to ensure
waveguide 350 is defined over the portion of bend section 20 where
the core refractive index n.sub.c becomes so small that the
overclad is required for continued optical confinement of guide
wave 352.
[0112] The various embodiments of the present disclosure have a
number of advantages. The laser written waveguides include portions
that reside near the glass outer surface and provide strong optical
confinement for tight bends, enabling extremely low profile optical
interface devices and thus compact photonic devices. The mode field
diameter and the aspect ratio of the laser written waveguide can
change along the length of the waveguide to minimize coupling
losses between optical fibers and active devices with dissimilar
mode field sizes.
[0113] The degree of optical confinement, especially at the bend
section, can be modified by including a low-index coating or an
overclad layer.
[0114] In an example, the optical interface device disclosed herein
has an all-glass construction that enables the use of light-curable
adhesives. The laser written waveguides build flexibility into the
assembly process, enabling excellent alignment of written
waveguides to existing fiber arrays. The laser written waveguide
process can accommodate optical fiber arrays and active devices on
arbitrary 1D and 2D pitches, allowing the optical interface device
to function as a pitch converter or furcation element.
[0115] The support member can be made of glass and fabricated using
low-cost glass drawing processes known in the art. The optical
interface device can also be made by heating and bending
inexpensive fusion drawn glass sheets into desired profile prior to
waveguide writing. The laser written waveguides allow low precision
attachment of optical fiber array to bent glass component, reducing
manufacturing cost and complexity.
[0116] The use of glasses that are CTE-matched to silicon ensures
low stresses at mechanical joints under thermal cycling for high
reliability interconnections to silicon-based devices. The use of a
polymer overclad layer on the glass bend section protects the glass
bend surface from damage during assembly and in use, minimizing
optical scattering losses and potential mechanical failure by
preventing scratches in the glass surface.
[0117] It will be apparent to those skilled in the art that various
modifications to the preferred embodiments of the disclosure as
described herein can be made without departing from the spirit or
scope of the disclosure as defined in the appended claims. Thus,
the disclosure covers the modifications and variations provided
they come within the scope of the appended claims and the
equivalents thereto.
* * * * *